We are determining amino acid sequences of ovomucoids and ovoinhibitors of various avian species. We also isolate, characterize and sequence Kazal family inhibitors from various tissues (egg white, pancreas, blood, semen) of several reptiles (alligator, snake, turtle) and birds (chicken, turkey). We sequence those deemed of special evolutionary interest. We examine in detail the inhibition of various serine proteinases (elastase I and II, chymotrypsins A, B, C, subtilisin, lytic proteinase) by highly purified ovomucoid third domains. We attempt to explain: a) the gross evolution of various Kazal type inhibitor domains, b) the detailed evolution of ovomucoid domains from an avian species to another avian species, and c) the consequences on inhibitor activity and specificity of single amino acid changes that occur during the course evolution.